Wikipedia Traffic Since Adding Nofollow
Many have wondered what effect it might have on your site's search traffic to tag nearly every outbound link with the "nofollow" attribute. According to Alexa, Wikipedia hasn't suffered. Looking at the following graph, with the red vertical line showing the rough date on which Wikipedia added "nofollow" attributes to its outbound links, one could draw the superficial conclusion that a global "nofollow" addition has had neither particularly positive effects (i.e., the "PageRank hoarding" theory) nor negative effects (i.e., the "if Wikipedia doesn't trust its links then Google won't trust Wikipedia's pages" theory):
While this graph supposedly reflects all traffic, Hitwise suggests that Wikipedia gets over 50% of its traffic from Google. So theoretically, a large hit in Google traffic would appear on this chart.
It might be more accurate to look at Wikipedia traffic from the source itself. Pulled from this page (a very cool resource), we see a traffic graph (measured in bits/sec -- not visits or pageviews) that similarly confirms no traffic loss following the "nofollow" implementation:
An interesting footnote: This graph shows incoming traffic (e.g., new articles, picture uploads, comments, etc.) below the X-axis (the red horizontal line near the bottom), while outgoing traffic (typical file requests, etc.) are above the X-axis. It's very cool that they show this.